Various cleaning systems for filter installations are known which permit cleaning of soiled filters. Filter installations are used in numerous industrial processes and serve fundamentally for cleaning/dedusting a fluid flow which is conducted through such filter installations. In the interior, the filter installations generally have a plurality of filtering bags which free the fluid flowing therethrough from contaminants, such as, for example, dust particles. In most cases, the fluid flow is conducted through the filtering bags from the outside inward, and therefore the contaminants are deposited on the outside of the filtering bags. After a certain operating period, the filtering bags are soiled and have to be cleaned.
To start with, the filters were cleaned by shaking or knocking by means of shaking devices operated by motor or manually. With the aid of such methods, the filtering bags are vibrated and the filter cake detaches from the outer surface of the filters because of said movements. However, the mechanical cleaning has resulted in high stressing of the filtering bags, and therefore the service life thereof is shortened.
In order to reduce the stressing of the filtering bags, a transition is made to cleaning the filtering bags with an air flow which is supplied at a positive pressure in relation to the filter pressure. Particularly cleaning flows which are applied in a pulsed manner to the filtering bags are particularly suitable for efficiently cleaning the filtering bags. Therefore, filtering bags are predominantly cleaned nowadays with the pulse-jet method. In such a method, the cleaning medium is conducted from the inside outward (in the counterflow direction to the fluid flow) through the filtering bags, as a result of which the filter cake is detached from the outer surface of the filtering bags and collected in a dust collection container. The first pulse-jet methods subjected the bag filters to a pulsed cleaning flow at a constant cleaning pressure. However, it has been demonstrated that effective cleaning of the filtering bags is not possible if the cleaning pressure is not matched to the filter pressure.
In order to carry out the cleaning, it is ensured that the pulse which is applied to the bag filters by the cleaning air does not excessively stress the bag filters. A decisive factor here is the positive pressure at which the cleaning medium is supplied. If the positive pressure of the cleaning pressure is selected to be too high in relation to the filter pressure, the service life of the bag filters is reduced. In the worst case, the bag filter may even be destroyed by an excessive positive pressure. However, in the event that the positive pressure has not been selected to be sufficiently high, the bag filter is not efficiently cleaned. Accordingly, the cleaning pressure is regulated in such a manner that it lies within a predetermined range in relation to the filter pressure, and therefore the cleaning pressure suffices in order effectively and gently to clean the bag filter.
U.S. Pat. No. 5,837,017 describes a cleaning device which detects the differential pressure between the “clean side” of the bag filter and the “dirty side” of the bag filter with pressure sensors. The cleaning device in U.S. Pat. No. 5,837,017 is based on the assumption that the soiling of the bag filters depends on the size of the pressure difference measured between the “clean side” and “dirty side”. If the size of the pressure difference measured has reached a defined threshold value, the control unit recognizes that the filter is clogged and opens a pressure valve which supplies the cleaning nozzles with cleaning air. The pressure valve is thus controlled by the control unit in such a manner that the cleaning pressure is matched to the pressure difference in the filter. Accordingly, the cleaning pressure is matched to the degree of soiling. In the case of the installation described, the pressure both on the “clean side” and on the “dirty side” is measured with the aid of pressure sensors and forwarded to a control unit.
However, the pressure sensors on the “dirty side” of the bag filter are continuously exposed to contaminants, and therefore reliable detection of the differential pressure is problematic. Since the filter cleaning involves a highly automated process, the maloperation of one of the two pressure sensors can be established from “the outside” only with difficulty. Therefore, a maloperation is only noticed, if at all, when it is already too late, i.e. when the filtering bags either are prematurely torn, or when the filtering bags are clogged up to such an extent that an increased pressure loss occurs in the filter installation.
Furthermore, the installation controls the cleaning via the differential pressures and not via the absolute pressures. If the filter pressure increases, for example because of external influences, the bag filter can no longer be cleaned since the required cleaning pressure can no longer be supplied.